U.S. patent application Ser. No. 449,618 filed Mar. 11, 1974 (priority: Switzerland No. 3619/73 of Mar. 13, 1973) relates to a process for the manufacture of heat-curable synthetic resins, based on reaction products of maleic anhydride with mixtures of polybutadiene, unsaturated hydrocarbon resins and unsaturated fatty acid glyceride ester, which can be diluted with water and are suitable for the electrophoretic coating process, characterised in that a mixture consisting of:
A. 20 - 60% BY WEIGHT OF A POLYBUTADIENE HAVING AN AVERAGE MOLECULAR WEIGHT OF 750 - 2,000 AND AN IODINE NUMBER BETWEEN 300 AND 450,
B. 10 - 60% BY WEIGHT OF A HYDROCARBON RESIN OF PREDOMINANTLY ALIPHATIC STRUCTURE, HAVING A VISCOSITY BETWEEN 30 AND 800 CP (measured in 70% strength solution in toluene at 20.degree.C) and an iodine number between 160 and 400, consisting of
B.sub.1. 20 - 80% by weight of polyisoprene, PA1 B.sub.2. 5 - 35% by weight of cyclopentadiene and/or dicyclopentadiene, PA1 B.sub.3. 5 - 50% by weight of further mono-unsaturated or poly-unsaturated hydrocarbons with 5 - 6 C atoms which are of predominantly aliphatic or cycloaliphatic structure and PA1 b.sub.1. 20-80% by weight of polyisoprene, PA1 b.sub.2. 5-35% by weight of cyclopentadiene and/or dicyclopentadiene, PA1 b.sub.3. 5-50% by weight of further mono-unsaturated or poly-unsaturated hydrocarbons with 5-6 C atoms which are of predominantly aliphatic or cycloaliphatic structure and
C. 5 - 40% BY WEIGHT OF A FATTY ACID GLYCERIDE ESTER, WHEREIN THE FATTY ACID RADICAL CONTAINS 16 - 18 C atoms and the fatty acid glyceride ester has an iodine number between 140 and 220, is pre-polymerised by heating to 200.degree. - 270.degree.C until the reaction mixture of components a, b and c, which has an initial viscosity of about 100 - 300 sec (measured according to DIN 4 sec 53,211), displays viscosities between 500 and 2,000 sec (measured according to DIN 4 sec 53,211), the resulting mixture is then reacted with
D. 10 - 20% BY WEIGHT OF MALEIC ANHYDRIDE AT 180.degree. TO 190.degree.C until no further free maleic anhydride is present and
E. IN THE RESULTING ADDUCTS THE ANHYDRIDE GROUPS PRESENT ARE OPENED BY HYDROLYSIS WITH WATER OR BY ALCOHOLYSIS WITH THE AMOUNT OF MONOHYDRIC ALCOHOLS WITH 1 - 4 C atoms required to form the half-ester.
The process of U.S. patent application Ser. No. 449,618 gives heat-curable synthetic resins which can be diluted with water, are suitable for the electrophoretic coating process and are distinguished by very good resistance to salt spray, good throwing powers, good film hardness and low sensitivity of the rinsed, but not yet stoved, films to drops of water. However, a continuing disadvantage of the binders described in U.S. patent application Ser. No. 449,618 is that at high deposition potentials, say above 300 volt, the binders tend to excess-coating, that is to say coating thickness of 30 .mu. and above are obtained on the metal sheets to be coated.
An object of the present invention is to provide a process for the manufacture of synthetic resins of this type which when used as binders no longer show this disadvantage and which, for these reasons, are distinguished by even better values of the throwing power and excellent voltage resistance; this means that the breakdown voltage, measured in volt, during electrical deposition is relatively high. The binders manufactured according to the invention should be capable of deposition over a wide voltage range without a danger of excess-coating, that is to say in coating thicknesses of 20 - 22 .mu..
In the process of the present invention, this is achieved by using, additionally to the components (a) to (d) of U.S. Patent Application Ser. No. 449,618, a component (f), which consists of an adduct of an .alpha.,.beta. -unsaturated aliphatic dicarboxylic acid to a resin acid, it being possible for these adducts to be partially or completely esterified with saturated polyhydric alcohols.